As a communication using light, a fiber optics communication using optical fibers is known. The fiber optics communication uses a low-loss optical fiber and a semiconductor laser, and is developing as a long distance communication network. However, in the fiber optics communication, due to loss in optical fibers and wavelength dispersion or the like, the communication distance is limited up to about 100 km.
Relative to such optical fibers, free space, especially cosmic space, becomes a high-quality communication path with less absorption, scattering and dispersion or the like of light. Optical communication technology using this free space as a communication path can realize a communication in the distance of several thousand km to several tens of thousands of km, such as a communication of inter artificial satellite in orbit around the earth, or a communication between an artificial satellite and an ground station.
Other than the optical communication, as a communication using free space as a communication path, there is a microwave communication which uses microwaves. Incidentally, a beam diffusion angle of electromagnetic waves such as light and microwaves used for a communication is proportional to the wavelength of electromagnetic waves to be used. The wavelength of light is several hundred nm, and this is very short with about one part several thousandth to one part several hundred thousandth of the wavelength (1 mm-0.1 m) of microwave used for a satellite communication. For this reason, relative to the microwave communication, the optical communication can narrow down a beam to be used, can make energy density of communication beam having reached an opposite device high, and can establish a communication system with high energy efficiency.
Thus, the optical communication can establish a communication system with high energy efficiency, however, it needs alignment of the beam direction with high accuracy because a beam diffusion angle is narrow.
Further, in the optical communication, because the communication distance is long, large displacement of beam at a communication destination occurs caused by a slight fluctuation in position and angle of communication devices due to a vibration or the like. Therefore, it is difficult to maintain a long-distance optical communication while fixing the direction of the beam even it is between artificial satellites each other in geostationary orbit of the earth.
Therefore, a capturing device of the optical communication is maintaining communication by providing a tracking mechanism which follows the partner according to the change in the direction of the communication partner or the displacement of own device.
Further, in a communication between a geostationary satellite and an artificial satellite moving in orbit around the earth, in a communication between artificial satellites moving in orbit around the earth, or in a communication between an artificial satellite moving in orbit around the earth and a ground station, the capturing device of the optical communication also needs capturing operation to find out the communication partner because the communication partner appears in and deviates from the communication possible range.
For this reason, the capturing device of the optical communication using free space is equipped with a capturing mechanism to perform capturing operation for finding out the communication partner and a tracking mechanism to follow the partner which is moving.
Particularly, in a communication such as between an artificial satellite and a ground station, the capturing device of the optical communication needs to support two different requirements, one is moving nearly 180-degree range of half of the sky and the other is direction alignment less than 1-degree to a device locating several thousand km away. It is difficult to realize such requirements by one mechanism. Accordingly, the generally used capturing device of the optical communication is realizing by providing a coarse capturing mechanism unit which has a large movable range but slow operation speed and a fine capturing mechanism unit which has a fast response speed but a narrow movable range. (For example, refer to patent documents 1 and 2.)
Incidentally, the capturing device finds the direction of communication partner using a receiving optical beam from the partner. Also, the capturing device transmits a transmission beam from own device based on the partner's direction found by the receiving optical beam. However, because the communication distance is long in the optical communication, even if a high-speed optical beam is being used, the position of the partner is changed during the time between when the partner transmitted the beam and when the beam of this side reaches the partner. Therefore, it needs compensation of the direction for sending out a transmission beam based on the partner's direction found by a reception beam in consideration of position change of the partner. An angle of the difference in directions of a reception beam and a transmission beam is called aberration. The capturing device of the optical communication equipped with an aberration compensation mechanism is used in order to compensate the aberration of a transmission beam. (For example, refer to patent document 3.)
The capturing device of the communication device equipped with such mechanism performs capturing operation first for finding out a direction of the partner in order to communicate.
A rough direction or the like in orbit of a satellite can be predicted in advance to some extent. However, because a diffusion angle of beam used for usual communication is narrow, it is difficult to predict a direction of the communication partner with accuracy as much a diffusion angle of beam. Therefore, the capturing operation for finding out the partner's direction is performed.
As this capturing operation, for example, there is one which performs the capturing operation using beacon light which is large in output power and also large in a diffusion angle compared with a communication beam. (For example, refer to patent document 4.)
One example of a capturing device using beacon light which is generally used is shown in FIG. 8.
The capturing device 8 of captured side irradiates with a beacon light having a wide diffusion angle from a beacon optical system 51. The beacon light here has a diffusion angle covering predicted angular range where the partner of a capturing side. Further, because the beacon light needs to be irradiated with quite a wide diffusion angle compared with such a beam used for an ordinary communication and also it secures a predetermined energy density when it arrives at the partner, it needs a high output power optical system. The beacon light outputted from the beacon optical system 51 is irradiated to the partner's device (not shown), at that time, it is irradiated as the beacon light which is keeping an enough diffusion angle to the partner's device using an optical antenna 52 and a coarse capturing mechanism unit 11.
A capturing controller 53 of the capturing device 8, which is the side of performing capturing operation, first, aligns the coarse capturing mechanism unit 11 with the prediction direction of the partner using a coarse capturing control signal 34. On the other hand, the capturing controller 53 sets a fine capturing mechanism unit 12 to face an initial direction using a fine capturing control signal 35. In this state, the capturing controller 53 waits for the beacon light from the partner's device. The beacon light from the partner's device is received in a capturing sensor 32 via the coarse capturing mechanism unit 11, the optical antenna 52, the fine capturing mechanism unit 12 and a beam splitter 16. As far as the beacon light has an enough diffusion angle and a high intensity, the capturing device 8 can always receive such beacon light in the capturing sensor 32. The capturing sensor 32 outputs a capturing sensor signal 33 which indicates the direction of the received beacon light to the capturing controller 53. The capturing controller 53 controls the direction of the fine capturing mechanism unit 12 so that the center of the beacon light may be aligned with the center of the capturing sensor 32 using the fine capturing control signal 35 from the capturing sensor signal 33. Further, the capturing controller 53 can operate the coarse capturing mechanism unit 11 using a coarse capturing control signal 34 as necessary.
In contrast, the capturing controller 31 can also perform the capturing operation by scanning a transmission beam.
Further, a transmission beam of a sending end device will be a reception beam at a receiving end device. In the description below, there may be a case that both of beams are called scanning beams as a matter of convenience in order to avoid complication.
An example of scanning operation using a scanning beam which is generally used is shown in FIG. 9.
In case of this example, a device of one side (a device X in FIG. 9) performs scanning the whole movable range using a scanning beam, and the capturing operation is performed with detecting the scanning beam at a device of the other side (a device Y in FIG. 9). The capturing operation performed by the device Y will not be completed by only one scanning operation of the device X. Therefore, it is needed to repeat the scanning operation of the device X more than once.
At a stage when the device Y has completed the capturing operation, this time, the device Y performs the scanning operation using a scanning beam, and the device X performs the capturing operation. Further, because the capturing operation of the device Y has been completed at this stage, the scanning beam from the device Y surely reaches the device X. Therefore, the capturing operation performed by the device X is completed in a short time compared with the capturing operation by the device Y.
However, in such a capturing device, because it needs to scan the whole scanning range more than once using a narrow scanning beam, it needs a long time for the scanning operation. As a result, the capturing time at the partner's device also becomes long.
Accordingly, in order to reduce the capturing time, as shown in FIG. 10, a capturing device adopting a method by which the scanning operation and the capturing operation are to be performed alternately at both of devices is used.
In the capturing method in the capturing device shown in FIG. 10, first, a device of one side (a device X in FIG. 10) performs the scanning operation (scan 1) of a scanning beam. The other side device (a device Y in FIG. 10) performs the capturing operation to the scanning operation. The device Y can narrow the scanning range of own device (the device Y) down to some extent based on this capturing operation. The device Y performs the scanning operation (scan 2) in the range having been narrowed down.
The device X performs the capturing operation to scan 2 performed by the device Y, and when the capturing operation having been completed, this time, the device X performs scan 2 which the scanning range has been narrowed down to the device Y similarly. The device Y performs the capturing operation to scan 2 performed by the device X. Using scan 2, the device Y can detect more accurate direction of the partner than scan 1, and can further narrow the scanning range down.
Because the direction of the partner is narrowed down mutually with narrowing the scanning range down in this manner, the capturing device described with reference to FIG. 10 realizes the capturing operation in which the capturing time has been reduced compared with the capturing operation in the capturing device described with reference to FIG. 9.
Further, in such a capturing device, in order to reduce misalignment between a transmission beam and a reception beam, it is also performed to form coaxial optical axes. (For example, refer to patent document 5.)    [Patent document 1] Japanese Patent No. 2518066 (Japanese Patent Application publication No. 1991-152490)    [Patent document 2] Japanese Patent Application Publication No. 2001-203641    [Patent document 3] Japanese Patent Application Publication No. 1995-307703    [Patent document 4] Japanese Patent Application Publication No. 1998-233738    [Patent document 5] Japanese Patent Application Publication No. 1990-180311